1. Field of the Invention
This invention relates to a method of and a system for operating a squeeze plunger in a die cast machine for improving the quality of a die cast product.
2. Description of the Related Art
With a die cast machine, there has heretofore been adopted such a technique that, after a cavity of a mold is filled up with a molten metal, the interior of the cavity is pressed by means of a squeeze plunger device, thereby causing plastic flows in a molding material.
A known injection unit in a die cast machine provided with a squeeze plunger device is shown in FIG. 3 for example. This injection unit 1 includes an injection plunger 4 connected to a piston rod 3 of an injection cylinder 2, and arranged to permit the injection plunger to perform moving strokes in two stages including a low speed injection region S.sub.1 and a high speed injection region S.sub.h in an injection sleeve 6 of a mold assembly 5.
Fixed onto the piston rod 3 is a dog 7, in the vicinity of which is disposed a limit switch 8. A positional relationship between the dog 7 and the limit switch 8 is detected so as to detect a position of stroke of the injection plunger 4, whereby a timer, not shown, is actuated to operate the squeeze plunger.
However, the conventional system suffers from the following problems.
That is, adjustment of the positional relationship between the dog 7 and the limit switch 8 to detect the position of stroke of the injection plunger 4 is very delicate, so that a considerably high skill level is required for the accurate adjustment. Furthermore, adjustment of the timing to operate the squeeze plunger depends upon the detection of the position of stroke of the injection cylinder 4, whereby the squeeze plunger is operated regardless of the actual state how a molten metal is filled and the like, so that improvements in the quality of a die cast product cannot necessarily be achieved. Moreover, in the conventional system, the state where the molten metal is filed up completely is uniformly and presumably grasped on the basis of positional relationship between the dog 7 and the limit switch 8, and the actual state of the filled-up molten metal is not fed back, so that it is very difficult to cope with the changes in the various conditions of molding, and the like.